The present invention relates generally to load shedding systems, facility management systems or demand limiting systems. In particular, the invention relates to load shedding systems utilizing predictive logic for employing peak demand shaving operations for energy bill minimization.
As is well known in the art, a facility management system can limit peak demand by turning off non-essential electrical equipment in a building. Owners or operators of commercial buildings are billed for electric power based in significant part upon the peak demand of energy consumed during a billing period. Thus, by limiting peak demand, owners or operators can minimize electric bills.
Load shedding systems are generally known in the art. Most load shedding systems employ relatively unsophisticated control systems for shedding loads. The less sophisticated control systems make shedding determinations based solely on current electrical demand data, and are disadvantageous because they are prone to a significant amount of on-off exercising. On-off exercising, or short cycling, refers to the turning on and off of loads in a short period of time.
On-off exercising makes load shedding systems inefficient. For instance, if such a load shedding system determines that too much electricity is being used, it will shed a load. If the load is large enough, the system will determine that more electricity can be used and will allow the load to be turned back on. Such cycling could be perpetual. As the cycle continues, electricity is wasted and the user is annoyed as the load is continually restarted.
The random nature of electrical consumption further complicates the situation. For example, people in buildings are constantly turning on and off lights and other appliances in a random manner. The demand profile of a building changes randomly as a result of this activity. After a load shedding system restores a previously shed load and allows the energy demand to approach a predetermined target level, the random use of appliances could force the load shedding system to once again disconnect a recently restored load. Thus, the random nature of electrical consumption contributes to the on-off exercising problem.
Prior art systems attempt to solve the on-off exercising problem by providing separate shed limits and restore limits. The restore limit is set below the shed limit. Under this scheme, a load is shed at a particular shed limit and not restored until the consumption level falls below a restore limit. Although this prior art method provides some relief to the on-off exercising problem, such a system is prone to excessive shedding because loads are not restored until the consumption level falls below the restore limit, a level below the shed limit. Excessive shedding is disadvantageous because the operator is annoyed as appliances are shed unnecessarily.
Less sophisticated prior art systems are also unable to adapt to a specific building's random demand characteristics. In prior art systems, shedding decisions are based upon current electrical demand data, thus making automatic adaption to the building's random demand characteristics impossible.
More sophisticated prior art systems employ predictive control logic. Such systems periodically measure the total accumulated energy consumption from the beginning of a demand interval. A predicted end-of-interval demand is determined based on the rate of accumulated consumption measured and the amount of time left in the demand interval, and a predicted present demand is interpolated based upon the same two factors and the predicted end-of-interval demand. If the predicted present demand exceeds maximum desired demand, a load is shed. Although such systems project the total consumption over an interval, the projection is based only on past consumption during the current demand interval. Thus, such systems are prone to inaccuracy because extraordinary measurements at a few points in time can affect load shedding operations across an entire interval. Also, such systems are inaccurate because they do not adapt to a specific building's random demand characteristics. Such inaccuracy may lead to excessive shedding or to energy consumption beyond a pre-established target level. Such systems are also disadvantageous because they are susceptible to on-off exercising for the same reasons as described above in connection with less sophisticated prior art load shedding systems.